In the manufacture of tissue products such as bath tissue, a wide variety of product characteristics must be given attention in order to provide a final product with the appropriate blend of attributes suitable for the product's intended purposes. Among these various attributes, improving softness has always been a major objective for premium products. Major components of softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness.
Traditionally, tissue products have been made using a wet-pressing process in which a significant amount of water is removed from a wet laid web by pressing or squeezing water from the web prior to final drying. In particular, while supported by an absorbent papermaking felt, the web is squeezed between the felt and the surface of a rotating heated cylinder (Yankee dryer) using a pressure roll as the web is transferred to the surface of the Yankee dryer for final drying. The dried web is thereafter dislodged from the Yankee dryer with a doctor blade (creping), which serves to partially debond the dried web by breaking many of the bonds previously formed during the wet-pressing stages of the process. Creping generally improves the softness of the web, albeit at the expense of a significant loss in strength.
More recently, throughdrying has become a more prevalent means of drying tissue webs. Throughdrying provides a relatively noncompressive method of removing water from the web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from the forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until it is dry. The resulting dried web is softer and bulkier than a wet-pressed uncreped dried sheet because fewer papermaking bonds are formed and because the web is less dense. Squeezing water from the wet web is eliminated, although subsequent transfer of the web to a Yankee dryer for creping is still used to final dry and/or soften the resulting tissue.
While there is a processing incentive to eliminate the Yankee dryer and make an uncreped throughdried tissue, attempts to make throughdried tissue sheets without using a Yankee dryer (uncreped) have heretofore lacked adequate softness when compared to their creped counterparts. This is partially due to the inherently high stiffness and strength of an uncreped sheet, since without creping there is no mechanical debonding in the process. Because stiffness is a major component of softness, the use of uncreped throughdried sheets has been limited to applications and markets where high strength is paramount, such as for industrial wipers and towels, rather than for applications where softness is required, such as for bath tissue, premium household towels, and facial tissue in the consumer market.